1. Technical Field
The present invention relates generally to burning gas, and more particular to a gas-burning appliance, which has high performance and decorative flames, and a gas fireplace.
2. Description of Related Art
A conventional direct-vented gas fireplace intakes and exhausts air in a naturally balanced way, with the exhaust port and the intake port horizontally or vertically connected to the combustion chamber, and communicating with outside. The indoor air is completely isolated from the combustion chamber, which makes the direct-vented gas fireplace the safest fireplace for now. Since the exhaust port and the intake port both communicate with outside, the exhaust pipe and the intake pipe are typically designed in a pipe-in-pipe way for easier installation. In other words, the vent line has an outer intake pipe surrounding a smaller coaxial inner exhaust pipe. The outer pipe also communicates with the intake passage located on the rear side of the furnace. The intake passage communicates with outside, and is adapted to intake fresh air into the combustion chamber through one or multiple intake ports. The inner pipe communicating with the combustion chamber is adapted to exhaust the high-temperature waste air generated by combusting out of the firebox. The combustor is provided in the combustion chamber in the firebox. With the heat generated by the combustor while combusting, the air in the combustion chamber would be heated and expanded, which makes the air go up and exit the combustion chamber through the exhaust pipe due to the stack effect. Meanwhile, the enclosed combustion chamber would have negative pressure inside, which sucks the outside fresh air into the combustion chamber to provide oxygen necessary for continuous combustion. In order to make the gas fireplace show nice flaming visual effect and provide heat radiation, a transparent glass cover would be provided at the front side of the firebox, so that a user could see and feel the light and heat of the burning flame inside the firebox through the glass cover. Except the front side which is provided with the glass cover, an outer casing is provided around the firebox by a certain distance to separate the high temperature of the firebox from the building, wherein the outer casing could be located near an outer wall of the building, which reduces the space required for installation. The space between the high-temperature firebox and the outer casing could exchange heat with the indoor air, while the space between the bottom side of the firebox and the outer casing could be used to receive a control valve and a control module, and sometimes even a fan is received therein to enhance convection, which facilitates heat exchange between the firebox and the indoor air. In this way, the heating efficiency could be improved, and the indoor temperature could be increased more quickly. The structure of the fireplace mentioned herein can be seen in the U.S. Pat. No. 4,793,332, titled “DIRECTED-VENTED GAS FIREPLACE.”
However, a good working direct-vented gas fireplace must meet several design requirements and regulations, including: (1) High performance: Since the intake and exhaust ports are both provided outdoors, the efficiency of heat usage has to be improved to comply with relevant laws and regulations. If either the exhaust temperature or the flow of the directed-vented gas fireplace gets too high, the performance of the fireplace would be decreased. (2) Nearly complete combustion: Though complete combustion is impossible in reality, the more it gets near complete combustion, the less carbon monoxide, hazardous material, and black smoke would be exhausted. Generally, the degree of complete combustion is not measured merely based on the absolute value of generated carbon monoxide, but is measured relative to the scale of combustion, wherein the scale of combustion could be represented by the amount of carbon dioxide. Therefore, the cleanness of combustion is usually evaluated by the relative ratio of CO and CO2. If the ratio of CO and CO2 is less than 0.004, the combustion is usually considered complete. The less this ratio is, the less amount of black smoke is generated. (3) Types and colors of flame: A fireplace has to mimic the visual effect of burning woods, which has mostly yellow-orange flame, to satisfy the aesthetic requirement of decorative flame. Colorless or blue flame could not meet the visual requirement of decorative flame. (4) Compatible with all kinds of fuel: Consumer fireplaces may be installed in many different regions, and therefore, one single model of fireplace usually has to be both compatible with natural gas (NG) and liquefied petroleum gas (LPG), and has to operate properly no matter it is horizontal or vertical direct-vented, or even in other conditions of actual use. Furthermore, fuel in each region may be somewhat different. Therefore, a fireplace has to not only meet the above requirements, but also be compatible with fuel of different compositions. (5) Compatible with large scale of combustion: To further improve the compatibility, one single model of fireplace must be compatible with large scale of combustion, and also meet the above requirements.
However, the above requirements tend to conflict with each other. For example, while lowering the exhaust temperature and flow to improve the thermal efficiency, the amount of intake air would be insufficient, leading to incomplete combustion and generating excessive carbon monoxide and black smoke. On the other hand, if the combustion is nearly complete, the flame would be colorless or blue, which fails to show the yellow-orange color visually required for decorative flame. Furthermore, it is not easy to have one single model of fireplace compatible with natural gas and liquefied petroleum gas of different components in different regions at the same time. The natures of natural gas and liquefied petroleum gas are inherently different. For example, natural gas requires less air supply than liquefied petroleum gas does. So it is possible that one fireplace combusts well with natural gas, but combusts incompletely with liquefied petroleum gas.
It's hard to solve the above problems at once, which usually takes more than one single means. This is because that, in the combustion chamber of a fireplace, the waste gas generated by combusting would form high-temperature airflow in the firebox, and flows toward the exhaust port at the top of the firebox. Since the cross-sectional area of the exhaust port is much less than that of the upper part of the combustion chamber, only small part of the high temperature airflow could successfully pass therethrough, while most of the uprising heated gas would be stopped by the wall of the top of the firebox, and turn downward to form a circulation. As a result, heat energy would be accumulated in the firebox, and then transferred into the room through the heat exchange ongoing outside the firebox. The amount of heat energy accumulated in the firebox could affect the efficiency of using energy. If the high-temperature gas is exhausted out of the firebox too quickly, the efficiency would be reduced; on the contrary, if it is exhausted too slowly, the outside air would be hindered from flowing into the firebox, which is not conducive to complete combustion.
In addition, while the outside air is guided into the firebox through the intake port, if the gas supply port of the combustor is far from the flame, the inflowing air and the high-temperature airflow formed by the waste gas of combustion tends to interfere with and blend into each other to create turbulence. Such condition would not only affect the exhaust of waste gas of combustion, but also lower the oxygen concentration in the air around the burning flame. Therefore, the supply of the amount of oxygen required for complete combustion would not be effectively controlled. Especially when the scale of combustion is expanded, the high temperature would further enhance the convection in the combustion chamber, which mixes more inflowing air into the waste gas of combustion, and more likely leads to incomplete combustion.
Prior art such as U.S. Pat. No. 4,793,332, titled “DIRECTED-VENTED GAS FIREPLACE”, discloses a continuous pusher gas fireplace with high performance, which exhausts small amount of carbon monoxide (CO) and nitride (NOx), and lowers the exhaust temperature and exhaust speed to improve the thermal efficiency by optimizing the air/fuel ratio.
U.S. Pat. No. 5,016,609, titled “DIRECT VENTED MULTI GLASS SIDE FIREPLACE”, discloses a high-performance continuous pusher gas fireplace which is further provided with glass on lateral sides. Said gas fireplace increases the flow of exhaust and intake air through a flow guide means. In addition, a heat exchange structure with extended surface area is provided at the top of the firebox to improve the thermal efficiency.
U.S. Pat. No. 5,452,708, titled “UNIVERSAL HORIZONTAL-VERTICAL (H-V) DIRECT-VENTED GAS HEATING UNIT”, discloses a high-performance continuous pusher gas fireplace compatible with horizontal and vertical air communication. In order to control the air/fuel ratio, the passage and the flow guide plate are arranged to make multiple intake ports located together and below the combustion tube, whereby the oxygen concentration on the combustion surface could be increased. A stop plate is further provided in front of the exhaust port at the top of the firebox to control the trace of exhausting the high-temperature waste gas.
U.S. Pat. No. 5,947,113, titled “DIRECT VENT GAS APPLIANCE WITH VERTICAL AND HORIZONTAL VENTING”, discloses a high-performance continuous pusher gas fireplace compatible with horizontal and vertical air communication. The passage does not directly communicate with the high-temperature firebox. A stop plate is further provided in front of the exhaust port at the top of the firebox to control the flow trace of the high-temperature waste gas.
U.S. Pat. No. 6,432,926, titled “DIRECT VENT FIREPLACE WITH BAFFLE, DIRECTIONAL EXHAUST AND VENT AIR COLUMN”, discloses a continuous pusher gas fireplace, which has a stop flow plate provided in front of the exhaust port of the firebox to increase the area to be heated, and has an airway to guide air to the bottom of the firebox. The thermal efficiency could be improved due to the heat exchange on the surface of the firebox is hindered.
Though the designs disclosed in these patents are different at adding different types of separators and flow guide plates in the combustion chamber, and at arranging the intake passage differently, they still have something in common. One is that either the traces of exhausting the high-temperature waste gas are all arranged in a way that the flow trace of the high-temperature waste gas becomes longer, or the areas for heat exchange at the high-temperature portion at the top of the combustion chamber are increased to improve heat exchange efficiency, and to evenly decrease flow speed, which prevents the high-temperature waste gas from causing excessive disturbance and circulation in the combustion chamber, and prevents the intake air from being excessively mixed into the waste gas of combustion. Another common aspect is that the intake ports of the combustion chamber are drawn near and are distributed roughly at the bottom of the burning appliance to increase the oxygen concentration in the flow field near the flame of the burning appliance, which facilitates complete combustion. Some of the disclosures even reduce the area of the intake passage which directly contact with the high-temperature firebox, which lowers the temperature of the intake air, and increases the efficiency of drawing in the intake air.
Though the current technology and designs could provide a certain benefit, it is not common to see a product integrating the forms of flame with the burning appliance, and the flow field in the combustion chamber and the amount of intake air are less seen to be precisely controlled. In light of this, while trying to comply with relevant laws and regulations, the use of a product might be limited.
As shown in FIG. 1 and FIG. 2, a conventional gas-burning appliance 1 is a long tube 10, which is linear or curved, and has a plurality of exhaust orifices 102 provided along a major axis thereof. An end of the tube 10 is adapted to accept gas to flow therein to perform a primary gas-mixing. After the primary gas-mixing, the gas would flow out through the exhaust orifices 102. While burning gas, the conventional gas-burning appliance 1 fails to effectively control the secondary air required for combustion. Therefore, the height of the flame generated from the exhaust orifices 102 could be effectively increased. Even if the amount of gas supply is raised to try to increase the height and the visibility of the flame, the outcome would not be apparent.
This is because that, by providing more gas supply to the exhaust orifices 102 to try to increase the height of the flame, the turbulence in the flow field near the exhaust orifices 102 would worsen, for the flow speed and heat energy are increased. Turbulence is a kind of flowing state of fluid. At low velocities, the fluid tends to flow without lateral mixing, and adjacent layers slide past one another, wherein the moving direction of molecules is the same as the direction of flow. Such phenomenon is called laminar flow, wherein no cross-currents perpendicular to the direction of flow. If the velocity is increased to a certain extent, molecules will move perpendicular to the direction of flow, creating many irregular tiny eddies in the flow field. Such phenomenon is called turbulence, which facilitates heat transfer or adequate mixture.
Laminar flow is helpful to generate wide yellow-orange flame which is more visible, and turbulence is helpful to mix the flammable gas and the nearby air during combustion. However, combustion requires certain conditions and reaction speed. Over-mixing combustion-supporting air tends to generate colorless or blue flame, to produce nitride (NOx), or to cause excessive flow speed in some parts, which is not conducive to complete combustion. These conditions all lower the visibility of the flame, and make the flame flicker discontinuously. Therefore, increasing the amount of gas supply would not effectively enhance the visibility of the flame, nor effectively enhance the visibility or scale of the wide yellow-orange flame.
In a gas fireplace, the turbulence generated in the enclosed firebox would enhance the disturbance and convection of airflow. Especially when the scale of the flame is expanded, the air with high oxygen concentration drawn from outside tends to be interfered by the turbulence. In such condition, it's hard to control the right combustion conditions. Therefore, the conventional gas-burning appliance 1 might not be perfect, and still has room for improvement.